Asynchronous successive approximation analog-to-digital converter and related methods and apparatus

ABSTRACT

An ultrasound device including an asynchronous successive approximation analog-to-digital converter and method are provided. The device includes at least one ultrasonic transducer, a plurality of asynchronous successive-approximation-register (SAR) analog-to-digital converters (ADC) coupled to the at least one ultrasonic transducer, at least one asynchronous SAR in the plurality having a sample and hold stage, a digital-to-analog converter (DAC), a comparator, and control circuitry, wherein a DAC update event following at least one bit conversion is synchronized to a corresponding DAC update event of at least one other ADC in the plurality of ADCs.

BACKGROUND

Field

The present application relates to ultrasound devices having asuccessive approximation analog-to-digital converter.

Related Art

Ultrasound devices may be used to perform diagnostic imaging and/ortreatment. Ultrasound imaging may be used to see internal soft tissuebody structures. Ultrasound imaging may be used to find a source of adisease or to exclude any pathology. Ultrasound devices use sound waveswith frequencies which are higher than those audible to humans.Ultrasonic images are made by sending pulses of ultrasound into tissueusing a probe. The sound waves are reflected off the tissue, withdifferent tissues reflecting varying degrees of sound. These reflectedsound waves may be recorded and displayed as an image to the operator.The strength (amplitude) of the sound signal and the time it takes forthe wave to travel through the body provide information used to producean image.

Many different types of images can be formed using ultrasound devices.The images can be real-time images. For example, images can be generatedthat show two-dimensional cross-sections of tissue, blood flow, motionof tissue over time, the location of blood, the presence of specificmolecules, the stiffness of tissue, or the anatomy of athree-dimensional region.

SUMMARY

According to an aspect of the present application, there is provided anapparatus, comprising at least one ultrasonic transducer, a plurality ofasynchronous successive-approximation-register (SAR) analog-to-digitalconverters (ADC) coupled to the at least one ultrasonic transducer, atleast one asynchronous SAR in the plurality having a sample and holdstage, a digital-to-analog converter (DAC), a comparator, and controlcircuitry, wherein a DAC update event following at least one bitconversion is synchronized to a corresponding DAC update event of atleast one other ADC in the plurality of ADCs.

According to an aspect of the present invention, there is provided amethod of operating an ultrasound device having a plurality ofultrasonic transducers and a plurality of asynchronoussuccessive-approximation-register (SAR) analog-to-digital converters(ADC), each ultrasound transducer being respectively coupled to an ADC,each asynchronous SAR in the plurality having a sample and hold stage, adigital-to-analog converter (DAC), a comparator, and control circuitry,the method including converting, in response to a first clock signal, afirst bit of one ADC, updating a DAC in the one ADC in response to theconverting; and updating a DAC in another ADC in the plurality of ADCsin response to the updating a DAC in the one ADC.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and embodiments of the application will be describedwith reference to the following figures. It should be appreciated thatthe figures are not necessarily drawn to scale. Items appearing inmultiple figures are indicated by the same reference number in all thefigures in which they appear.

FIG. 1 is a block diagram of an ultrasound device including ananalog-to-digital converter, according to a non-limiting embodiment ofthe present application.

FIG. 2 is a block diagram of an asynchronous successive approximationanalog-to-digital converter, according to a non-limiting embodiment ofthe present application.

FIG. 3 is a graph illustrating the time evolution of seven controlsignals associated with the asynchronous successive approximationanalog-to-digital converter of FIG. 2, according to a non-limitingembodiment of the present application.

FIG. 4 is a graph illustrating the time evolution of a possible bitsequence as converted by the asynchronous successive approximationanalog-to-digital converter of FIG. 2, according to a non-limitingembodiment of the present application.

FIG. 5 illustrates the steps of a method to perform analog-to-digitalconversion, according to a non-limiting embodiment of the presentapplication.

DETAILED DESCRIPTION

The inventors have recognized and appreciated that the power consumptionassociated with analog-to-digital converters may be greatly improved byremoving the need for power-hungry high-speed clock circuits.

Aspects of the present application relate to an asynchronous successiveapproximation analog-to-digital converter (ADC) that does not requirethe use of high-speed clock signals to govern the timing associated withthe successive conversion. Accordingly, each iteration of the successiveconversion process may be triggered by the completion of the previousiteration.

Furthermore, the inventors have recognized and appreciated that thespeed associated with analog-to-digital converters may be greatlyimproved by removing the need for time-constraining clock circuits. Theconversion speed of typical analog-to-digital converters is limited bythe repetition rate of the clock signal used to time the circuit.

Aspects of the present application relate to an asynchronous successiveapproximation analog-to-digital converter (ADC) that does not requirethe use of high-speed clock signals to govern the timing associated withthe successive conversion. Accordingly, the conversion speed may besolely limited by the delay caused by the circuitry necessary to performthe conversion.

The aspects and embodiments described above, as well as additionalaspects and embodiments, are described further below. These aspectsand/or embodiments may be used individually, all together, or in anycombination of two or more, as the application is not limited in thisrespect.

FIG. 1 illustrates a circuit for processing received ultrasound signals,according to a non-limiting embodiment of the present application. Thecircuit 100 includes N ultrasonic transducers 102 a . . . 102 n, whereinN is an integer. The ultrasonic transducers are sensors in someembodiments, producing electrical signals representing receivedultrasound signals. The ultrasonic transducers may also transmitultrasound signals in some embodiments. The ultrasonic transducers maybe capacitive micromachined ultrasonic transducers (CMUTs) in someembodiments. The ultrasonic transducers may be piezoelectricmicromachined ultrasonic transducers (PMUTs) in some embodiments.Further alternative types of ultrasonic transducers may be used in otherembodiments.

The circuit 100 further comprises N circuitry channels 104 a . . . 104n. The circuitry channels may correspond to a respective ultrasonictransducer 102 a . . . 102 n. For example, there may be eight ultrasonictransducers 102 a . . . 102 n and eight corresponding circuitry channels104 a . . . 104 n. In some embodiments, the number of ultrasonictransducers 102 a . . . 102 n may be greater than the number ofcircuitry channels.

The circuitry channels 104 a . . . 104 n may include transmit circuitry,receive circuitry, or both. The transmit circuitry may include transmitdecoders 106 a . . . 106 n coupled to respective pulsers 108 a . . . 108n. The pulsers 108 a . . . 108 n may control the respective ultrasonictransducers 102 a . . . 102 n to emit ultrasound signals.

The receive circuitry of the circuitry channels 104 a . . . 104 n mayreceive the electrical signals output from respective ultrasonictransducers 102 a . . . 102 n. In the illustrated example, eachcircuitry channel 104 a . . . 104 n includes a respective receive switch110 a . . . 110 n and an amplifier 112 a . . . 112 n. The receiveswitches 110 a . . . 110 n may be controlled to activate/deactivatereadout of an electrical signal from a given ultrasonic transducer 102 a. . . 102 n. More generally, the receive switches 110 a . . . 110 n maybe receive circuits, since alternatives to a switch may be employed toperform the same function. The amplifiers 112 a . . . 112 n may betrans-impedance amplifiers (TIAs).

The circuit 100 further comprises an averaging circuit 114, which isalso referred to herein as a summer or a summing amplifier. In someembodiments, the averaging circuit 114 is a buffer or an amplifier. Theaveraging circuit 114 may receive output signals from one or more of theamplifiers 112 a . . . 112 n and may provide an averaged output signal.The averaged output signal may be formed in part by adding orsubtracting the signals from the various amplifiers 112 a . . . 112 n.The averaging circuit 114 may include a variable feedback resistance.The value of the variable feedback resistance may be adjusteddynamically based upon the number of amplifiers 112 a . . . 112 n fromwhich the averaging circuit receives signals. The averaging circuit 114is coupled to an auto-zero block 116.

The auto-zero block 116 is coupled to a time gain compensation circuit118 which includes an attenuator 120 and a fixed gain amplifier 122.Time gain compensation circuit 118 is coupled to an analog-to-digitalconverter (ADC) 126 via ADC drivers 124. In the illustrated example, theADC drivers 124 include a first ADC driver 125 a and a second ADC driver125 b. The ADC 126 digitizes the signal(s) from the averaging circuit114.

According to aspects of the present application ADC 126 may be asuccessive approximation ADC. Successive approximation ADCs convertcontinuous analog waveforms into digital representations by performing abinary search through all possible quantization levels. In someembodiments, an asynchronous successive approximation ADC is used.

While FIG. 1 illustrates a number of components as part of a circuit ofan ultrasound device, it should be appreciated that the various aspectsdescribed herein are not limited to the exact components orconfiguration of components illustrated. For example, aspects of thepresent application relate to the successive approximation ADC 126.

The components of FIG. 1 may be located on a single substrate or ondifferent substrates. For example, as illustrated, the ultrasonictransducers 102 a . . . 102 n may be on a first substrate 128 a and theremaining illustrated components may be on a second substrate 128 b. Thefirst and/or second substrates may be semiconductor substrates, such assilicon substrates. In an alternative embodiment, the components of FIG.1 may be on a single substrate. For example, the ultrasonic transducers102 a . . . 102 n and the illustrated circuitry may be monolithicallyintegrated on the same semiconductor die. Such integration may befacilitated by using CMUTs as the ultrasonic transducers.

According to an embodiment, the components of FIG. 1 form part of anultrasound probe. The ultrasound probe may be handheld. In someembodiments, the components of FIG. 1 form part of an ultrasound patchconfigured to be worn by a patient.

FIG. 2 illustrates successive approximation ADC 200, according toaspects of the present application. The ADC may comprise asample-and-hold circuit 210, a comparator 220, adigital-to-analog-converter (DAC) 240 and successive approximationregister (SAR) controller 230. The output of the ADC is a digitalrepresentation of the input analog signal comprising of a word of Nbits. N may have any value between 5 and 20.

According to aspects of the present application, the analog-to-digitalconversion performed by ADC 200 is an iterative process. In eachiteration the digital representation of the analog input voltage may befurther improved by successively decreasing an error signal.Furthermore, ADC 200 may operate in an asynchronous fashion, such thateach iteration is triggered by the completion of the previous iteration.

In some embodiments, ADC 200 may be coupled to one ultrasound transducerof a M×N array of ultrasound transducers, where M and N may assume anysuitable value. In some other embodiments, a single transducer may becoupled to a plurality of circuits of the same type as ADC 200. In yetsome other embodiments, ADC 200 may be fed by a signal that is obtainedby combining the signals transduced by a plurality of ultrasoundtransducers.

According to aspects of the present application, in successiveapproximation ADC 200, the conversion is performed one bit at a time,starting from the most significant bit (MSB) to the least significantbit (LSB). Accordingly, the conversion of the i^(th) bit of the sequenceoccurs at a non-predefined time and is triggered by the completion ofthe conversion of the i−1^(th) bit. By way of explanation, the timingassociated with the conversion of each bit is not governed by a clocksignal, as it would be the case for synchronous successive approximationADCs.

In some embodiments, the conversion of one bit is synchronous and istriggered by a clock signal, while all the other bits are asynchronous.In some embodiments, the most significant bit is the synchronous bit. Inother embodiments, at least two bits, but not all bits, are synchronouswhile all the other bits are asynchronous.

Sample-and-hold (S/H) circuit 210 may be configured to receive an analoginput signal. Sample-and-hold 210 may be single-ended or differential.During a specified period of time, sample-and-hold circuit 210 maycapture a portion of the voltage associated with the input signal.Subsequently, sample-and-hold 210 may hold the captured voltage to aconstant value. In some embodiments, sample-and-hold 210 may comprise aswitch and a capacitor (not shown). During the sample phase, the switchmay be in a “closed” state thus connecting the input voltage to thecapacitor. In this phase, the input voltage may charge or discharge thecapacitor for as long as the switch stays “closed”. In the hold phase,the switch may be in an “open” state thus disconnecting the inputvoltage from the capacitor. The charge stored in the capacitorthroughout the sample phase may result in a voltage across the capacitorthat is proportional to the input voltage. During this phase, thecapacitor may maintain the captured voltage at a constant level.However, the capacitor may be charged or discharged by its own leakagecurrents and consequently the stored voltage may change over time.Signal clks may be used to determine whether the switch ofsample-and-hold 210 is in “closed” or “open” state. In some embodiments,when clks is equal to a logic 1 the switch is “closed” and when clks isequal to a logic 0 the switch is “open”. However, the opposite logic mayalso be implemented. In some embodiments, the switch may be one or acombination of field effect transistors (FET), bipolar junctiontransistors (BJT) or any other suitable types of transistor.Furthermore, in some embodiments, the switch may follow an operationalamplifier configured as a buffer amplifier to charge and discharge thecapacitor.

The voltage acquired by sample-and-hold circuit 210 may be sent to oneinput port of comparator 220. The second input port of comparator 220may be connected to the output of DAC 240. In some embodiments, if theacquired voltage is greater than the DAC output voltage, then comparator220 may output a voltage corresponding to a “high” level or logic 1.Contrarily, if the DAC output voltage is greater than the acquiredvoltage, then comparator 220 may output a voltage corresponding to a“low” state or logic 0. However, any other suitable logic may beimplemented. In some embodiments, comparator 220 may comprise anoperational amplifier. In some embodiments, comparator 220 may be gatedby signal clkc. In the “gated” state, comparator 220 may be configuredto perform a comparison and output a voltage based on the input signals.In the “ungated” state, comparator 220 is not active and does notperform any comparison. In some embodiments, when clkc is equal to alogic 1 comparator 220 is gated and when clkc is equal to a logic 0comparator 220 is ungated. However, the opposite logic may also beimplemented.

Logic states 0 and 1 may represent any voltage as long as the voltage orvoltage range corresponding to a logic 0 is different from the voltageor voltage range corresponding to a logic 1. In some embodiments, logic1 corresponds to 1.8V and logic 0 corresponds to 0V. In someembodiments, logic 1 corresponds to any voltage between 0.5V and 5, andlogic 0 corresponds to any voltage between −5V and 1V, such that the tworanges do not overlap.

According to aspects of the present application, successiveapproximation register (SAR) controller 230 may comprise one or moreregisters and a logic circuit. One of the registers may store the mostrecent digital representation of the analog input voltage. The contentof the register may be continuously updated based on the result of thecomparison performed by comparator 220. The digital word contained inthe register may be transferred to DAC 230 that, in turn, may perform adigital-to-analog conversion. In some embodiments, the initial state ofthe register prior to the beginning of the analog-to-digital conversionmay be set such that the most significant bit (MSB) is set to 1 whileall the other bits are set to 0. In this scenario, DAC 240 may output ananalog signal equal to Vref/2, where Vref is the reference voltageapplied to DAC 240. However, any other suitable initial state may beimplemented. In some embodiments, DAC 240 may be configured to outputVref when a digital word containing all 1s is received and may befurther be configured to output 0V when a digital word containing all 0sis received. In some embodiments, DAC 240 comprises a chargedistribution circuit. DAC 240 may further comprise a bank of capacitorsdisposed in a single-ended or differential configuration.

In some embodiments, the output digital representation may be configuredto be equal to the input to DAC 240 as illustrated in FIG. 2. In someembodiments, the output digital representation may be stored in adedicated register of SAR controller 230.

According to aspects of the present application, the analog-to-digitalconversion performed by ADC 200 is an iterative process. In eachiteration the digital representation of the analog input voltage isfurther improved by successively decreasing an error signal equal to thedifference between the input signal and the DAC output signal.

The logic circuit of SAR controller 230 may be configured tosequentially scan through each bit of the N bits forming the digitalrepresentation of the analog input voltage. In some embodiments, duringthe first iteration the most significant bit is determined based on theresult of the comparison performed by comparator 220. By way of example,if the output of the comparison is a logic 1, corresponding to ascenario in which the acquired signal is greater than the DAC outputsignal, the most significant bit (MSB) of the shift register is setto 1. Once the state of the MSB is determined the logic circuit skips tothe following bit. The process continues until the least significant bit(LSB) is determined.

ADC 200 may operate in an asynchronous fashion, according to aspects ofthe present application. Each iteration may be triggered by thecompletion of the previous iteration. Control signals clks and clkc maybe generated by the logic circuit of SAR controller 230 in response toclock signal clk and signal adc_clk.

FIG. 3 illustrates a non-limiting example of a timing diagram accordingto aspects of the present application. Signal adc_clk may be used toinitialize the analog-to-digital conversion. In addition, clock signalclk may be provided to SAR controller 230. Clock signal clk may have arepetition rate between approximately 100 Hz and 10 GHz, betweenapproximately 1 KHz and 100 MHz, between approximately 1 MHz and 50 MHz.In some embodiments, an edge of adc_clk, for example a rising edge, mayinitiate the conversion. Subsequently, an edge of clk, for example arising edge, may trigger clks to switch to a logic 1. While clks isequal to 1, sample-and-hold circuit 210 may sample the analog inputsignal. Signal clks may remain in a 1 state for the duration of a clkcycle. In this case, when a second clk rising edge is provided, clks mayreturn to 0. However, clks may remain in a 1 state for any suitableamount of time. In some embodiments, the second edge of clks, forexample a falling edge, may trigger clkc to switch to a logic 1. Whileclkc is equal to 1, comparator 220 may compare the acquired signal tothe DAC output signal. Signal clkc may remain in a 1 state for anysuitable amount of time.

Each signal sel_i selects a bit of the register of SAR controller 230,where sel_0 selects the MSB and sel_N−1 selects the LSB. In someembodiments, when sel_i is set to 1, the i^(th) bit of the register maybe updated based on the result of the comparison performed by comparator220. In some embodiments, the MSB may be triggered by clk, for exampleby a falling edge of clk. By way of example, sel_0 may switch to 1, whena falling edge of clk is provided. In some embodiments, all other bitsexcept for MSB are triggered asynchronously. For example, when sel_0 isswitched to 1, an edge of sel_0, for example a rising edge, may triggera clkc pulse of any suitable duration, consisting of a rising edgefollowed by falling edge. The falling edge of clkc may in turn triggersel_1 to switch to a 1 state. Similarly, sel_1 may trigger clkc which inturn may trigger sel_2. The methods may continue until the LSB isreached. The delay between successive bits may be tuned, for example byadjusting the duration of a clkc pulse. However, any other suitabletechnique to delay bits can be used.

According to aspects of the present application, clock signal clk may beused to trigger only a subset of the digital word. For example, clk mayonly trigger the MSB while all other bits may be triggered by theprevious bit. Consequently, the requirements associated with therepetition rate of the clock signal may be relaxed compared tosynchronous successive approximation ADC.

FIG. 3 shows a non-limiting example of how control signals clk_adc, clk,clks, clkc and sel_i, where i may assume any value between 0 and N−1,may be used to drive ADC 200. However, any other suitable control signalmay be used, in substitution of or in addition to the aforementionedcontrol signals. All control signals may be edge-triggered by either arising edge or a falling edge, or may alternatively be pulse-triggered.

By way of example, FIG. 4 shows a non-limiting analog-to-digitalconversion of an input voltage V_(in), according to aspects of thepresent application. In the non-limiting example, a 8-bit representationof the analog input voltage is provided. However, any number of bits maybe used. In the non-limiting example, V_(in) may exhibit a voltagebetween V_(ref) and V_(ref)/2 and the ADC may be configured such thatthe initial DAC output voltage is set to V_(ref)/2. Accordingly, beforethe conversion is initiated at time t₀, the value of the register may beequal to “10000000” where the first digit represents the MSB. Between t₀and t₁, a comparison between V_(in) and V_(dac) may be performed, whereV_(dac) represents the DAC output voltage. In the non-limiting example,because V_(in) is greater than V_(dac), the MSB remains in a 1 state.The numeric table illustrated under the temporal chart, shows thecontent of the register after time t₁. The latest bit being updated isshown as underlined in the table. Between t₁ and t₂ a second comparisonmay be performed. In the non-limiting example, because V_(dac) isgreater than V_(in), the second bit remains in a 0 state. Between t₂ andt₃ a third comparison may be performed. In the non-limiting example,because Vin is greater than V_(dac), the third bit is set to 1. Theconversion may continue iteratively until the LSB is reached.

FIG. 5 illustrates a method to perform a digital-to-analog conversion,according to aspects of the present application. Method 500 may start atstep 502, for example when a rise edge of signal adc_clk is received bySAR controller 230. At step 504 the register may be set to “10000000”.In the non-limiting example, the digital representation may be performedwith an 8-bit long word. However, any number of bits can be used.Regardless of the length of the register, the MSB may be set to 1 andall other bits may be set to 0. At step 506 an edge of clks may bereceived by sample-and-hold 210, and the analog input voltage may besampled and stored. At step 508 comparator 220 may be gated throughsignal clkc triggered by an edge of clks. The triggering edge may be afalling edge. At step 510 comparator 220 may determine whether V_(in) isgreater than V_(dac) or vice versa. In the former case, the i^(th) bitmay be set to 1, otherwise the i^(th) bit may be set to 0. At step 514SAR controller 230 may determine if the i^(th) is the LSB. If the i^(th)is not the LSB at step 516 the i+1^(th) bit may selected by settingsel_i+1 to 1. The selection of the i+1^(th) bit may be performedasynchronously through an edge of clkc, as illustrated in FIG. 3. Atstep 518 a digital-to-analog conversion may be performed through DAC240. Subsequently, method 500 may perform another iteration, and theupdated value of V_(dac) may be compared to V_(in). Otherwise, if thei^(th) bit is the LSB, the conversion of the sampled analog voltage maybe completed. At step 520 the method determines if the analog-to-digitalconversion is done. If the analog-to-digital conversion is not done, themethod may continue from step 504, and a new sample of the analog inputmay be captured and converted.

Furthermore, the amount of time saved may be significant. In typicalsuccessive approximation analog-to-digital converters, the timenecessary to perform a conversion may be limited by the repetition rateof the clock used to time the circuit. In some embodiments, utilizing anasynchronous successive approximation analog-to-digital converter of thetypes described herein may provide a substantial time saving by removingunnecessary idle period of times spent waiting for the subsequent clockedge. In some embodiments, utilizing an asynchronous successiveapproximation analog-to-digital converter of the types described hereinmay provide up to a 10% time saving, up to a 25% time saving, up to a50% time saving, or any range or value within such ranges, in terms ofthe ADC.

In some embodiments, a plurality of successive-approximation-register(SAR) analog-to-digital converters (ADC) coupled to the ultrasonictransducer may be provided.

Having thus described several aspects and embodiments of the technologyof this application, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those of ordinaryskill in the art. Such alterations, modifications, and improvements areintended to be within the spirit and scope of the technology describedin the application. It is, therefore, to be understood that theforegoing embodiments are presented by way of example only and that,within the scope of the appended claims and equivalents thereto,inventive embodiments may be practiced otherwise than as specificallydescribed.

As described, some aspects may be embodied as one or more methods. Theacts performed as part of the method(s) may be ordered in any suitableway. Accordingly, embodiments may be constructed in which acts areperformed in an order different than illustrated, which may includeperforming some acts simultaneously, even though shown as sequentialacts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements.

As used herein, the term “between” used in a numerical context is to beinclusive unless indicated otherwise. For example, “between A and B”includes A and B unless indicated otherwise.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively.

What is claimed is:
 1. An apparatus, comprising: at least one ultrasonictransducer; a plurality of asynchronoussuccessive-approximation-register (SAR) analog-to-digital converters(ADC) coupled to the at least one ultrasonic transducer, at least oneasynchronous SAR ADC in the plurality having a sample and hold stage, adigital-to-analog converter (DAC), a comparator, and an SAR controllerthat receives a first clock signal and a second clock signal as inputsthereto; wherein the SAR controller is configured to generate a firstcontrol signal in response to the first clock signal that causes thesample and hold stage to sample an input analog signal thereto, and togenerate a second control signal in response to the second clock signalthat causes the comparator to compare the sampled input analog signal toan output signal of the DAC, with an output of the comparator configuredto generate a DAC update event corresponding to an individual bitconversion; and wherein conversion of a most significant bit (MSB) ofthe SAR ADC is triggered by the second control signal being responsiveto the second clock signal, and conversion of other bits of the SAR ADCis triggered by the second control signal being responsive to a previousbit conversion.
 2. The apparatus of claim 1, wherein the at least oneultrasonic transducer comprises an M×N array of ultrasonic transducers.3. The apparatus of claim 1, wherein conversion of more than one bit,but not all bits of the SAR ADC, is triggered by the second controlsignal being responsive to the second clock signal.
 4. A method ofoperating an ultrasound device having a plurality of ultrasonictransducers and a plurality of asynchronoussuccessive-approximation-register (SAR) analog-to-digital converters(ADC), each ultrasonic transducer being respectively coupled to anasynchronous SAR ADC, each asynchronous SAR ADC in the plurality havinga sample and hold stage, a digital-to-analog converter (DAC), acomparator, and an SAR controller that receives a first clock signal anda second clock signal as inputs thereto, the method comprising:sampling, in response to a first control signal, an input analog signalreceived by a sample and hold stage, the first control signal beinggenerated by the SAR controller in response to the first clock signal;converting, in response to a second control signal, a first bit of oneasynchronous SAR ADC; updating the DAC in the one asynchronous SAR ADCin response to the converting; and converting, in response to the secondcontrol signal, one or more additional bits of the one asynchronous SARADC and updating the DAC; wherein the second control signal is generatedby the SAR controller, the second control signal causing the comparatorto compare the sampled input analog signal to an output signal of theDAC, wherein conversion of the first bit of the one asynchronous SAR ADCis triggered by the second control signal being responsive to the secondclock signal, and conversion of the one or more additional bits of theone asynchronous SAR ADC is triggered by the second control signal beingresponsive to a previous bit conversion.
 5. The method of claim 4,wherein conversion of more than one bit, but not all bits of the SARADC, is triggered by the second control signal being responsive to thesecond clock signal.